Recently, in steam-power generation using a steam turbine, electricity is usually generated at 600° C. or less. In many cases, main components such as a turbine rotor and a rotor vane constituting the steam turbine and used under these conditions are formed of, for example, high Cr-steel (high-chrome steel and ferritic heat resistant steel) such as 12Cr steel.
Incidentally, in recent years, there has been a demand for generating electricity at conditions in which the steam is 700° C. or more in order to meet requirements for a reduction in the discharge of CO2 and further improvements in thermal efficiency. However, due to the insufficient high-temperature strength of the main components, it is not desirable to use ferritic heat resistant steel under these conditions.
Therefore, in order to ensure a higher high-temperature strength, a Ni-based alloy (nickel base alloy) is used for the main components. However, when the Ni-based alloy is used, there are disadvantages in that the main components may not be easily made in large sizes and the cost may increase.
In Patent Document 1 below, in order to both allow an increase in size and suppress cost of the turbine rotor, a turbine rotor is formed by joining a first member formed of a Ni-based alloy to a second member formed of high Cr-steel by welding. Then, the strength of the joint portion is ensured by using a Ni-based alloy with a specific composition.